Interesting air movement

[Dave W]

We've had a bunch of snow here lately and I haven't always been as diligent about brushing it off 100% most of the time (the truck is high) so there is always a little left.

I was driving down the interstate at normal speeds and the back window got bombarded with the left over snow from the roof. This tells me that there is a heck of negative pressure here causing turbulance which in turn causes drag and of course, this drag has a negative effect on gas/fuel mileage. I have a soft tonneau cover which helps it some. While this is not a whine on fuel mileage - I've acceped the fact that a 7400 pound 4x4 will never beat a Prius by a gas station - I am wondering if anyone has ever done any studies or testing on air dams/deflectors/streamlining on these "eco-terrorists nightmaressmilie " means of transportation. Any results?

 

[surewhynot]

I saw somewhere that when the bed is empty and the tailgate is shut, an air bubble is supposed to be trapped up by the cab creating kind of an air dam. I can't remember where I read that though.

 

[mrfixit]

Lunde kits don't just look nice, ya know... they also help cut the turbulence. it also comes from the sides, not just the front or the tailgate. Autoanything has them.

 

[surewhynot]

Are you talking about the ones that go on the bed behind the cab?

 

[mrfixit]

like this:

 

[O'Rattlecan]

That's always what I've heard recommended. The tailgate traps the air in, making a bubble that cuts down on turbulence, not add to it. What Tom posted will keep the same air circulating in there well.

Ryan

 

[Dave W]

I'll have to take a look at the Lunde stuff tho not sure I really like that particular look - and for sure wont work with the 5th wheel camper I'm looking at buying as a replacement for my conventional pull-behind trailer.

 

[mrfixit]

There are many variations of it. Even my own is different. I like the principle, though.

 

[blacksnapon]

The mythbusters on tv did a test on that. Both with a simulated (it was water) windtunnel, and with live testing with 2 equal trucks (one tailgate up, one tailgate down), to see which one got better mileage.

 

[mrfixit]

He's got the tonneau anyway... but how did that turn out?

 

[blacksnapon]

tailgate up----------better mileage. The water test was with a scale model, water was moving, they added oatmeal flakes I believe, and it showed the "bubble" very well.

 

[mrfixit]

yeah... I found it.
It is more fuel efficient to drive your pick-up truck with its tailgate down, rather than up.
busted
Driving with the tailgate down actually increased drag on the pick-up and caused it to consume fuel faster than the identical truck driven with the tailgate up. It was later revealed that the closed tailgate creates a locked vortex flow that created a smoother flow of air over the truck. With the tailgate down the trapped vortex was dissipated and the drag increased.
(This myth was revisited in episode 64 and re-busted. However, it was found that mesh tailgates are the most efficient configuration.)

 

[polarbear]

For some reason, folks think air flows over the top of a truck and into the bed. It doesn't- it tumbles and rolls. The tailgates of the newer trucks are shaped to actually assist in the process and reduce drag. The killer on trucks isn't the tailgate, it's the huge frontal area.

 

[Dave W]

For some reason, folks think air flows over the top of a truck and into the bed. It doesn't- it tumbles and rolls. The tailgates of the newer trucks are shaped to actually assist in the process and reduce drag. The killer on trucks isn't the tailgate, it's the huge frontal area.

Go back to my thread opener - there was nothing said about the air just flowing over the top - only that there is a negative pressure area moving snow (dust, mud, rain, etc.) up against the back window, The deflector shown in an earlier post will eliminate, to a point, the tumbling air flow. Also, the test by Myth Busters only showed one point of controlling air flow. And yes, the front end of a newer Super Duty is not much in the way of streamlined - especially since it has the aerodynamics of a barn door

 

[mrfixit]

When I first got my CDL, we were told that the PSI coming off the sides of the truck were directly proportionate to the total square inches displaced by the front of the vehicle, with most of it passing to either side, so with on oncoming wind, a truck can literally be pushing off nearly 600psi per side; The pressure generally does not increase, but the VOLUME increases with every MPH. More than this, when one is being passed by another, the dissimilarities in speed create increased vortices, or turbulence.